Remove or reconnect the unused C# qualified constant-pattern helper

[Widthdom]

Problem and priority

P3 — posting order 10/10. Audit candidates: D10.

The private 17-line helper IsCSharpQualifiedConstantPatternReferenceSite has only its declaration in the indexed repository. Verify its intended role and either remove obsolete code or restore a demonstrably missing call path in one small PR.

Reproduction and evidence

dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll unused --actionable --exclude-tests --json --compact --limit 30 --db .cdidx/codeindex.db
dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll search IsCSharpQualifiedConstantPatternReferenceSite --exact-substring --json --search-fields path,line,snippet --limit 30 --db .cdidx/codeindex.db

The actionable unused query and MCP unused_symbols both identify the helper with medium confidence. The exact-substring search returns only its declaration at src/CodeIndex/Indexer/References/Languages/CSharpReferenceExtractor.QualifiedPatterns.cs:98 (lines 98–114 in the audited checkout).

Implementation instructions and cautions

• Check the current source and Git history before deciding whether the helper is obsolete; indexed absence is not proof of language-level dead code.
• Recheck conditional/generated/test and any string/reflection-based use. If the helper has no real consumer, remove only that member and leave live constant-pattern/reference logic intact.
• If history instead establishes a missing necessary call, demonstrate the wrong reference behavior and reconnect the smallest correct path with a focused regression test.
• Keep this scoped to this helper and its intended behavior. Do not turn it into a broad dead-code sweep, new analyzer, extraction redesign or unsupported performance claim.

Acceptance and validation

• Compile with the repository's warnings-as-errors policy and run the affected C# qualified/constant-pattern reference tests.
• Preserve positive/negative constant-pattern and member-read cases. Add a behavioral regression only if behavior changes; do not add a test that merely asserts the private name was deleted.
• Record whether the result is behavior-preserving cleanup or a restored missing path, with the supporting evidence.

Prior issues

Follow-up to closed #5328 (four different declaration-only members) and its earlier cleanup #5283. The prior #5328 targets were AppendDependencyGeneratedFilter, TryCountTopLevelParameters, IsLexicalPathEqualOrParent and DefaultReadBufferSize. This is an additional remaining helper, not evidence that any deleted member returned.

Baseline and delivery

Observed during the 2026-09-12 dogfood audit on macOS arm64, using repository-built cdidx 1.49.0 / Debug net8.0 at 36f2e68cb0d7f17bc9591d01ad3045abe870da9b. The build passed with zero warnings/errors. The root database and workspace-manifest freshness checks were healthy, with index and reference-graph completeness true. Counts/timings describe that checkout. Validation during discovery used focused CLI/MCP/LSP reproductions, not the full test suite.

Implement one focused PR for this issue. Follow AGENT_GUIDE.md and the relevant workflows, use the repository-built cdidx for discovery, preserve supported .NET 8/.NET 9 and cross-platform behavior, and avoid new runtime dependencies outside repository policy. Add focused behavioral tests for changed contracts, update affected help/documentation, and include a bilingual changelog.d/unreleased/ fragment for user-visible changes.